The phylogenetic position of Myxobolus carnaticus (Myxozoa, Myxosporea, Bivalvulida) infecting gill lamellae of Cirrhinus mrigala (Hamilton, 1822) based on 18S rRNA sequence analysis

Myxozoans are an economically important group of microscopic parasites best known for the diseases they cause in commercially important fish hosts. The classification of myxosporeans is generally based on the morphology of their myxospores. Without molecular data, it is very difficult to identify new or existing species. DNA sequence information is therefore, a prerequisite to taxonomic and phylogenic studies of myxosporeans. In the present study, a myxozoan parasite, Myxobolus carnaticus, infecting the gill lamellae of mrigal carp, Cirrhinus mrigala, was characterized by the 18S rRNA gene sequence. The DNA sequence of M. carnaticus clustered phylogenetically with other gill infecting Myxobolus spp. of freshwater clades, forming a dichotomy with closely related M. pavlovskii (HM991164) that infects the gill lamellae epithelium of silver carp, Hypophthalmichthys molitrix with 95% similarity. Evolutionary pair-wise distances among M. carnaticus and other species of myxosporeans indicated high genetic diversity among myxosporeans. The present study demonstrated that tissue tropism, host specificity and habitat play important roles in phylogenetic relationships among myxozoan species.


INTRODUCTION
Myxoboliasis, caused by myxosporean parasites, is one of the most widely distributed fish diseases. Myxosporean species of the genera Myxobolus, Thelohanellus, MBRC http://mbrc.shirazu.ac.ir 126 Henneguya, Kudoa, Myxidium, Myxosoma and several others have been found to be the causative agents of the disease [1]. These are generally histozoic parasites of freshwater fish; nevertheless, coelozoic myxozoan parasites from marine fish are also available [2]. Heavy carp mortality associated with gill myxoboliasis has raised concern among fish farmers [3]. The current classification of myxosporeans is based on myxospore morphology, owing to the fact that their vegetative stages usually do not possess sufficient features for classification. Important characteristics are the size and shape of the myxospores and polar capsules, the number of shell valves, polar capsules and sporoplasms, the position of polar capsules on the plane of the suture and their location in the spore, the presence of surface ridges, projections and envelopes in the spore, characteristics of the polar filament, etc. Nevertheless, the use of such method makes morphological characterizations of similar myxosporeans very difficult. To resolve this issue, molecular taxonomy is implemented using the small subunit ribosomal DNA sequence. Smother et al. [4] were the first to use ssrDNA sequence analysis to study the phylogeny of Myxozoa. Since then, the inclusion of sequence information has become a necessary requirement for taxonomic and phylogenic studies of myxosporeans. In this report, the molecular characterization and phylogeny of Myxobolus carnaticus infecting the gill lamellae of Cirrhinus mrigala are presented.

MATERIALS AND METHODS
For the purpose of the present study, the Myxobolus species infecting the inner base of gill lamellae of Cirrhinus mrigala (Hamilton, 1822), collected from Garia (Lat. 22°27'59''N; Long. 88°24'18''E), South 24 Parganas District, West Bengal, India, during the routine survey of carp parasitic diseases in 2013, was characterized by morphometric and molecular techniques. Myxosporean identification was performed according to Lom and Arthur [5]. Details on spore collection, slide preparation, polar filament extrusion, iodinophylic vacuoles detection, staining, permanent mounting and micrometry are described in Mondal et al. [6]. The universal eukaryotic primers -ERIB1, 5´-ACC TGG TTG ATC CTG CCA G-3´ and ERIB10, 5´-CTT CCG CAG GTT CAC CTA CGG-3´ [7] were used for the amplification of the 18S rRNA gene by Eppendorf Master cycler Pro S. Molecular characterization of the Myxobolus species, viz., DNA extraction, PCR amplification, purification of amplicon, sequencing, electrophoresis and data analysis was done as described in Mondal et al. [6] and Abraham et al. [8]. The nucleotide sequence generated in the present study was then deposited in the NCBI GenBank database under accession number KF796620.
Phylogenetic analysis was performed on a selection of 18S rRNA gene sequences comprising the new sequence (KF796620) and 25 additional sequences from closely related species of freshwater and marine origin available in the NCBI GenBank database using the basic local alignment search tool (BLAST). Sequence alignment was performed by Multiple Sequence Comparison by Log-Expectation (MUSCLE) program [9] using MEGA6 software [10]. Bayesian Tree Estimation was applied to generate a phylogenetic tree using MrBayes One Model [11] in TOPALi v2 software [12]. A total MBRC http://mbrc.shirazu.ac.ir 127 of 100,000 generations were taken for the phylogenetic tree. Genetic distance analyses were conducted using the Kimura 2-parameter model [13] in MEGA6 [10]. Included codon positions were 1 st + 2 nd + 3 rd + Noncoding. All positions containing gaps and missing data were eliminated.
The phylogenetic tree established by Bayesian Estimation for the new sequence ( Fig. 2) was similar to that of Fiala [15]. The novel DNA sequence of M. carnaticus clustered phylogenetically with other gill infecting Myxobolus spp. of freshwater clades, and formed a dichotomy with carp gill infecting M. pavlovskii (HM991164) with a high node support. All marine species comprising of Ceratomyxa sp. (DQ333431), Kudoa funduli (AF195510), Myxidium gadi (GQ890675), and Myxidium maxi (KF179055) were phylogenetically clustered as a separate lineage (Fig. 2). Thelohanellus spp. clustered separately in the tree, and so did the other myxosporeans with different tissue specificity. The observed wide range in the evolutionary pair-wise distances among M. carnaticus and other species of myxosporeans, measured by Kimura-2 parameter algorithm (Table 1)  This report is the first of its kind to desribe the molecular phylogeny of M. carnaticus (KF796620) infecting the gill lamellae of C. mrigala. Earlier, we characterized M. cuttacki KF465682 [16] and M. orissae KF448527 [8] infecting carp gills and T. caudatus KC865607 [6] infecting caudal fins of carp from India. Myxosporeans are characterized as host, organ and tissue specific organisms [17]. According to Eszterbauer [18], site specificity is an important factor in myxozoan phylogeny.
Our study also demonstrated that tissue tropism, host specificity and habitat play important roles in phylogenetic relationships among myxozoan species. As the list of myxosporean parasites described from India is growing [19], molecular data on these parasites are needed to establish a genetic data-base which would help understand their taxonomy, phylogeny and genetic diversity among different ecological niches in India.